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has anybody seen any reports on testing the effects of different width tires on the same vehicle. im talking different widths(but close to the same diameter) of the same brand and model tire on the same car with the same suspension setup. i did some searching on here and google but couldnt find anything.

I had 225 Khumo MX's while a local friend had 205 MX's, both on GC's and I was signifcantly faster in corners... Theres more variables than that of course, but wider tires = your tires don't give out as quickly in a corner = faster speeds in a corner.

SCC did an article about that a few years ago. They put a series of +1, +2 and +3 packages (with the same model tire) on a Civic or something and tested it.

But really there are too many variables to say that just bolting on wider tires without changing anything else will make your car handle better. Start with the camber change inherent in a strut suspension. There's a reason guys who run monster-meat tires also run suspensions that limit travel with huge spring rates and sway bars.

Also, grip does not always equal perceived good handling. How many times do tires with higher absolute limits feel worse in the real world?

As always, no simple answer unless you limit your question so much that the answer doesn't matter.

It would be very difficult to get solid data for this. The results are going to vary so much that conclusions you draw will be unusable. I think it is enough to say that wider tires will give you more grip otherwise there wouldn't be people trying to cram 285's in the fenders of these cars.

I think about the only thing you have on your side right now is science. You'd be hard pressed to find someone do a real-world side-by-side comparison testing only tire widths(I'd assume different wheel widths as well).

From a simple physics standpoint, one would argue no difference.
f = u*N
or friction equals the coefficient of friction times the force pressing down on the tire

Of course it's not this simple. For one, we have heat. A wider tire has a greater capability to disipate heat and stay cool. As well, the wider tire also has less force per square inch applied to it. A 265 width tire has only 75% of the load carrying duties as a 205 width tire. Less work, less heat, less wear.

The larger contact patch also provides more area to generate traction, larger area of contact with the road and more biting edges.

As well, the coefficient of friction isn't linear as more force is applied. As more force applied, there is less and less of a gain. You actually gain a greater benifit with the less force per square inch.

It's much like making a car faster by lightening the car or lower the CG of the car. Anything you can do to minimize the work the tire needs to do helps. A wider tire just spreads out the force more and makes the work of the tire that much easier.

All cars would have super wide tires if there were no negatives to using a wide tire. Despite the benifits, there are problems as well.

One obvious one is camber. When you make a tire wider, it becomes more camber sensitive. This is kind of dependent on the wheel width and aspect ratio too. You can run a 235/70/R14 on a 6" rim and it may not care so much about camber due to all the flex available. You definately wouldn't like the handling, but camber wouldn't be a problem. A more appropriate setup would be 235/40/R19 on a 8" rim (just picking random numbers, nothing specific to our cars). This would handle a lot better and is sized more appropriately. However, the low flex available would make the contact patch very sensitive to camber changes.

You also have the physical limitations of the car. You can only fit so wide of a tire in the wheel wells before you start rubbing on things, either fenders or the struts/coilovers. With the front tires, you may even rub in the wheel wells themselves during a turn.

There's comfort as well. Handling was kind of metioned above. You can range from a narrow high aspect ratio setup to a wide, low aspect ratio setup and vary the handling and ride qualities of the tire. On a narrow rim, you'll produce an increasingly sloppy handling feel as you widen the tire. So there's limitations by the rim as well, both physical and desired handling.

A big issue with wide tires is the idea of deformable surfaces. A wider tire isn't always benifitical on all surfaces. On a solid surface like asphalt, yes, wider is better. However, on a deformable surface like gravel, dirt, or snow narrower is actually better. Since these surfaces are deformable, i.e. they change shape due to pressure. We can use a narrower tire and actually compress and dig into the surface gaining large amounts of grip, at times more so than a tire can generate even on asphalt. Think ruts and grooves here. A narrower tire even helps on wet surfaces. The higher contact patch pressure helps force more water out from under the tire...rather than floating over it.

Then there's gas milage and friction. A wider tire has more grip and generates more rolling friction. As well, its wider swath also has to be pushed through the air, resulting in more air friction as well. Milage drops can be quite noticable. This is also dependent on the particular tire, it's rubber compound, and even its air pressure.

In the end, how wide a tire is best depends on what surface(s) you plan on running on. Where do you need traction the most? What trade-offs are match your needs best?

It's not as simple as wider is better. There is a best match for you. It may not be the same match as another person. Wider is generally better, but how much traction in other areas are you willing to give up?

This isn't much of a quantitative response. But, it's tough to give exact answers as there are many different results depending on many factors, things like car used, suspension setup, outside temperature, surface conditions(cleanliness/type), even the tester's driving behavior will all affect the results.

Throw your physics book out the window. In the real world, pressure, temperature, tread compound and sidewall composition all matter.

Don't say: "For a given [insert variable here]" because in the real world everything is a variable.

Did I say they didn't??? All I said was, most people think a wider tire gives them a larger contact patch. It reality doesn't. If they can run lower pressures, they'd get a larger one, but then again, you could do the same thing with your skinny tire to get a larger patch. , therefore, eliminating pressure as a variable in that case is proper.

We AREN'T talking about sidewall stiffness, etc...

Whats your deal??? I know that there are a number of things that go into a tires performance. But my statement stands,

....putting on a wider tire does NOT give you a larger contact patch, just one that is a different shape...

You can't be saying that ^^^^ is not true, it is, and thats all I said...

contact patch area is determined by pressure, wheel rim diameter, tread width and section height for a given load. the static contact patch area is (virtually) the same for the following sizes:

205/55x16
225/45x17
225/40x18

what changes as you lower the profiile of the tire, reducing it's section height and increasing it's tread width, is that you increase the width of the contact patch and shorten it's length. A shorter, wider contact patch will provide better dry cornering capability, while a longer, narrower contact patch can be better for resistance to aquaplaning and dealing with snow.

the contact patch shape becomes wider - the size (area) remains the same

a shorter, wider contact patch at the same slip angle begins to slip at the same distance from the tire's leading edge as a tire with a longer, narrower contact patch. However, the wider contact patch has more of its length in contact with the road than the narrower contact patch and therefore a larger portion of the overall contact patch area is creating grip. For the same load and slip angle, a wider contact patch generates more grip in dry road conditions.

of course, when you are actually using the tire as intended, the dynamic contact patch of the tire constantly changes shape depending upon the load forces acting on the tire. once you choose your tire the section height, tread width, and tire diameter become fixed. increasing the width of the wheel by .5" will increase the tire's section width - and by implication tread width - by .2" resulting in a wider contact patch. the remaining variable - tire pressure - becomes crucial in determing contact patch shape and therefore the performance of the tire.

for the track, research done by Jim Hall of Chapparal Can-Am racing fame along with Firestone led to a doubling in width of race tires in the mid-60's. Every time Firestone sent Hall a wider tire, he and his team tested them on consecutively wider wheels, going from 15x7 to 15x9. Each time the increase in tire width and wheel width led to greater cornering forces during skidpad testing. Then Firestone would create a new tire designed for the 9" wheel and Hall would increase wheel width again with the same results. The difference in grip was enough to force Hall to redesign his transmission to handle the greater loads. the end result was an increase in Can-Am race tire width from 6" to 12" in less than 18 months and the acceptance of the fact that a wider contact patch translates into better dry cornering and braking.

as far as street testing goes, in January of '04 Consumer Reports of all people tested plus-sized tire and wheel combinations and found that the wider wheels and tires improved wet/dry handling, emergency handling, wet/dry braking and amazingly enough ice handling! comfort, noise, snow and hydroplaning resistance all decreased in performance. in typical Consumer Reports fashion, the specific tires and the raw test data is left up to the imagination of the reader. The differences were not great in terms of percentage - less than 10% improvement - all we have to go on thanks to their testing methods. However, had they broken out dry handling and dry braking results from the wet the performance differences may have been more dramatic. the fact that the car will handle better in emergency situations should be enough reason for street applications. isn't that the real reason we buy better tires in the first place - to save our lives when the unexpected happens?

putting on a wider tire does NOT give you a larger contact patch, just one that is a different shape...

You can't be saying that ^^^^ is not true, it is

In the sterile world of theory, it is. In the real world, no one would ever keep all the other variables constant, so the old saw is wrong far more often than it's right.

So why not look at an actual case? Let's see what happens when you compare, say, a 205/55-16 RE92 on a stock wheel with a stiff sidewall, soft compound, max performance 245/40-17 on a 17x8.5 wheel--a typical WRX big meat upgrade. Then you optimize the tire pressure for maximum grip without peeling off the rim. Then measure the contact patch. Who knows--maybe it'll be bigger, maybe smaller, maybe exactly the same, but you won't find the answer from theory.

In the sterile world of theory, it is. In the real world, no one would ever keep all the other variables constant, so the old saw is wrong far more often than it's right.

So why not look at an actual case? Let's see what happens when you compare, say, a 205/55-16 RE92 on a stock wheel with a stiff sidewall, soft compound, max performance 245/40-17 on a 17x8.5 wheel--a typical WRX big meat upgrade. Then you optimize the tire pressure for maximum grip without peeling off the rim. Then measure the contact patch. Who knows--maybe it'll be bigger, maybe smaller, maybe exactly the same, but you won't find the answer from theory.

its not theory - it is physics and there are laws, not theories. The contact patch area is the same, the shape changes. Its simple, your car weighs, for the sake of argument, 3000lbs and is perfectly balanced so that each wheel supports the same weight. Thus each tire is supporting 750 pounds. Say your running 30PSI in every tire, 750/30 = 25 square inches of contact patch for each tire. There is no magic here, if there is less weight on a tire the contact patch area shrinks, or if there is more weight it grows.
The contact patch area is based on the weight and tire pressure, period. It doesn't get bigger or smaller from the size of the tire but the shape of the contact patch will vary based on the tire's size and tread pattern. It will get bigger and smaller as the loads change, there is no way around it since its part of newton's laws.

speed rating can have a big play as well. slower speed rated tires are not as "square" as higher speed rated tires. also tires that have a rolled edge will handle better at the limit than a tire that is flat.